A liquid crystal display device and an EL (electroluminescence) display device which respectively have a liquid crystal element and a light-emitting element in each pixel are represented as a typical example of a display device. These display devices have display elements such as a liquid crystal element or an organic light-emitting element (hereinafter, referred to as a light-emitting element) in each of the plurality of pixels formed over a substrate. A liquid crystal element and a light-emitting element have a liquid crystal and a layer including an organic compound (hereinafter, referred to as an organic layer), respectively, between a pair of electrodes and are operated by applying a voltage or supplying current between the pair of electrodes.
Apart from a liquid crystal element, current flows in an organic compound included in a layer (hereinafter, referred to as an EL layer) sandwiched between a pair of electrodes in a light-emitting element. Therefore, the organic compound is oxidized or reduced during operation of the light-emitting element and may exist in a state having an electric charge. Additionally, an excited state is formed by recombination of these active species. Such active species readily react with other organic compounds or impurities such as water and oxygen which enter the light-emitting element because they have a higher reactivity than an electrically neutral state or a ground state. The products formed as a result of the reaction influence the characteristics of the light-emitting element and cause a decrease in efficiency and lifetime of the light-emitting element.
As a method for suppressing the aforementioned deterioration of the characteristics, it has been known to form a protection film (passivation film) over a light-emitting element. As disclosed in Japanese patent application publications No. 2014-154450 and 2009-266922, it is possible to prevent or suppress the reduction of the efficiency and lifetime of the light-emitting element because the entrance of the impurities into the light-emitting element can be prevented or suppressed by the protection film.